Combination fluidized catalytic cracking (FCC)-regeneration processes wherein hydrocarbon feedstocks are contacted with a continuously regenerated freely moving finely divided particulate catalyst material under conditions permitting conversion into such useful products as olefins, fuel oils, gasoline and gasoline blending stocks are well known. Such FCC processes for the conversion of high boiling portions of crude oils comprising vacuum gas oils and heavier components customarily referred to as residual oils, reduced crude oils, vacuum resids, atmospheric tower bottoms, topped crudes or simply heavy hydrocarbons and the like have been of much interest in recent years especially as demand has exceeded the availability of more easily cracked light hydrocarbon feedstocks. The cracking of such heavy hydrocarbon feedstocks which comprise very refractory components, e.g. polycyclic aromatics and asphaltenes and the like, capable of depositing relatively large amounts of coke on the catalyst during cracking, and which typically requires severe operating conditions including very high temperatures has presented problems associated with plant construction materials and catalyst impairment.
At present, there are several processes available for fluidized catalytic cracking of such heavy hydrocarbon feedstocks. A particularly successful and much preferred approach which avoids such problems as mentioned above is described, for example, in U.S. Pat. Nos. 4,664,778; 4,601,814; 4,336,160; 4,332,674; and 4,331,533.
In such processes, a combination fluidized catalytic cracking-regeneration operation is provided wherein catalyst regeneration is successively carried out in separate relatively lower and higher temperature regeneration zones each independently operating under selected conditions to provide hot, fully regenerated catalyst with very limited catalyst impairment per catalyst regeneration cycle. Such hot regenerated catalyst is then employed in the high temperature, highly selective catalytic cracking and simultaneous conversion of both high and low boiling components contained in heavy hydrocarbon feeds.
Due to the nature of heavy hydrocarbon feeds, cracking in such FCC processes as described above increases selectivity tending toward light cycle gas-oil and higher boiling materials production. These products are often employed as a component of diesel fuels and furnace oils preferably after hydrotreating or caustic treating. Catalytic cracking of such feeds, however, tends to oppose selectivity to lower boiling components for use as gasoline blending stocks, or as precursors for synthesizing gasoline blending stocks, especially those of higher octane values. It is believed that such competing effects arise in part due to carbon laydown on the catalyst as the catalyst travels through zones in the reactor. As the amount of carbon on the catalyst increases along the reaction path, the gasoline and light olefin selectivity from the heavy feed decreases. The higher the molecular weight of the feed hydrocarbon, the greater the carbon on catalyst competing effect because higher molecular weight components tend to contain more polynuclear aromatic compounds and asphaltenes which yield more coke upon initial cracking and vaporization than other compounds. Of the aromatic compounds, the polynuclear compounds not only crack at a slower rate, but will also have a much higher selectivity to C.sub.2 and lighter gases and coke production, while the mono- and di-aromatics and the alkyl side chains of naphthene components tend not only to crack at a faster rate, but also tend to exhibit a higher selectivity to gasoline and desired light olefins such as propylene, butenes, pentenes and hexenes. Therefore, as such heavier hydrocarbon feed undergoes cracking the heavier hydrocarbon feed components should be subjected to a reduced residence time at extremely high temperatures in order to limit the cracking thereof as much as possible to paraffinic side chains and mono- and di-aromatics in general to reduce excessive coke production. Alternatively, gasoline selectivity is optimized by more severe catalytic cracking operations of light hydrocarbon feeds, e.g. higher catalyst-to-oil ratios, longer residence times and relatively higher temperatures, than are desirable in the cracking of heavier feeds.
It often is desirable to operate FCC processes in a manner which maximizes the production of a given product or products, especially in the absence of competing effects such as mentioned above. For example, either one or both of the gasoline/light olefins and light cycle oil products may be desired in order to produce large quantities of high octane gasoline and gasoline precursors while simultaneously producing increased quantities of fuel oil distillates and diesel fuel. This is especially so in light of current environmental concerns which have necessitated a reduction in pollution by-products of combustion from automobiles from the use of leaded gasoline products. Therefore, unleaded gasoline blend stocks having a high octane number are much in demand. It would, therefore, be desirable to expand the operating envelope of such useful process as described above to increased selectivity to high octane material and light olefins while simultaneously selectively catalytically cracking economical heavy hydrocarbon feeds to heavy naphtha, and distillates or light and heavy cycle oils and higher boiling materials.
There are a number of ways of accomplishing these goals. The method described in U.S. Pat. No. 3,617,497 discloses segregating hydrocarbon feed and charging the relatively lower molecular weight feed fraction or fractions near the bottom of an elongated riser reaction zone and the relatively higher molecular weight feed fraction or fractions progressively further up the riser. Cracking of the lighter hydrocarbon feed in the absence of heavy hydrocarbon feed is thus accomplished on a low carbon content catalyst to maximize gasoline selectivity. Although feed residence times can be established in such a process by controlling the total charge rate of hydrocarbon to the riser, catalyst-to-oil ratios and reaction temperatures are difficult to optimize for maximum gasoline and light cycle oil selectivity, respectively.
A more versatile method for optimizing cracking selectivity from relatively lower and higher boiling feeds is described by U.S. Pat. No. 3,617,496. In such a process, cracking selectivity to gasoline production is improved by fractionating the feed hydrocarbon into relatively lower and higher molecular weight fractions capable of being cracked to gasoline and charging said fractions to separate riser reactors. In this manner, the relatively light and heavy hydrocarbon feed fractions are cracked in separate risers in the absence of each other, permitting the operation of the lighter hydrocarbon riser under conditions favoring gasoline selectivity, e.g. eliminating heavy carbon laydown, convenient control of hydrocarbon feed residence times, and convenient control of the weight ratio of catalyst to hydrocarbon feed therein thereby affecting variations in individual reactor temperatures.
Other processes which similarly employ the use of two or more separate riser reactors to crack disimilar hydrocarbon feeds are described, for example, in U.S. Pat. No. 3,993,556 (cracking heavy and light gas oils in separate risers to obtain improved yields of naphtha at higher octane ratings); U.S. Pat. No. 3,928,172 (cracking a gas oil boiling range feed and heavy naphtha and/or virgin naphtha fraction in separate cracking zones to recover high volatility gasoline, high octane blending stock, light olefins for alkylation reactions and the like); U.S. Pat. No. 3,894,935 (catalytic cracking of heavy hydrocarbons, e.g. gas oil, residual material and the like, and a C.sub.3 -C.sub.4 rich fraction in separate conversion zones); U.S. Pat. No. 3,801,493 (cracking virgin gas oil, topped crude and the like, and slack wax in separate risers to recover, inter alia, a light cycle gas oil fraction for use in furnace oil and a high octane naphtha fraction suitable for use in motor fuel, respectively); U.S. Pat. No. 3,751,359 (cracking virgin gas oil and intermediate cycle gas oil recycle in separate respective feed and recycle risers); U.S. Pat. No. 3,448,037 (wherein a virgin gas oil and a cracked cycle gas oil, e.g. intermediate cycle gas oil, are individually cracked through separate elongated reaction zones to recover higher gasoline products); U.S. Pat. No. 3,424,672 (cracking topped crude and low octane light reformed gasoline in separate risers to increase gasoline boiling range product); and U.S. Pat. No. 2,900,325 (cracking a heavy gas oil, e.g. gas oils, residual oils and the like, in a first reaction zone, and cracking the same feed or a different feed, e.g. a cycle oil, in a second reaction zone operated under different conditions to produce high octane gasoline).